Seismic and vibration isolation system

ABSTRACT

The invention relates to A method for protecting a structure from seismic ground motion and forced vibration comprising the following steps:  
     preparing a horizontal bearing surface or a solid foundation on top of which the isolation system will be placed;  
     providing flexible damping elements of a group consisting of unreinforced rubber slabs and fiber-reinforced elastomeric mats;  
     each mat consisting essentially of a piece of a rubber mat reinforced with fibers extending parallel to the surfaces of the mat;  
     covering the horizontal bearing surface or the solid foundation with several layers of damping elements, each layer consisting of a plurality of elongated pieces of the mat arranged side by side;  
     erecting the structure by putting the lower surface of the structure directly upon the upper surface of an uppermost layer of damping elements.  
     Further the invention relates to an elastomeric seismic isolation system for bearing a structure supported by a foundation

FIELD OF THE INVENTION

[0001] The invention relates to improvements in the design, layout andmanufacturing process of seismic and vibration isolation systemsprotecting structures from seismic ground motion or induced forcedvibration. These devices are located between foundation and structure inthe case of seismic isolation of structures (buildings) or simplybetween the prepared natural grade (or foundation) and the bottom plateof a liquid filled structure when used in petrochemical facilities, e.g.an oil tank. When used as vibration isolation devices, these systemswill be placed underneath the vibrating machine or underneath a propersupport in the case of subways and trains.

BACKGROUND OF THE INVENTION

[0002] It is well known that structures can be protected againstearthquakes by flexible bearings, decoupling the supported structurefrom the ground motion so that vibrations are prevented from propagatinginto the structure.

[0003] Typical earthquake accelerations have dominant periods of about0.1-1 seconds (1-10 Hz) with maximum severity often in the range between0.2-0.6 seconds. Structures whose natural periods of vibrations liewithin the range of 0.1-1 seconds are therefore particularly vulnerableto seismic ground motions because they may be subjected to highacceleration. The main feature of seismic base isolation is to increasethe flexibility of the structure. A higher flexibility translates intoan increase in the natural period of the structure beyond that ofearthquakes, resonance and near-resonance are avoided and the seismicinduced forces are reduced. The increased flexibility also affects thehorizontal displacements of structures during seismic events. Excessivedisplacements may be balanced by introducing better and higher dampingproperties to the isolation system. Conventional seismic designsolutions based on structural strengthening provide sufficientstiffness, deformability and energy-dissipating locations throughout thestructure to withstand the forces and dissipate the energy generated byearthquakes. The peak acceleration in the structure is often greaterthan the peak acceleration of the driving ground motion. On the otherhand, seismic base isolation limits the effect of earthquakes, since aflexible base decouples the structure from the horizontal ground motionand the structural response accelerations are usually less than theground accelerations. The addition of other energy dissipating devices(viscous dampers) may further reduce the energy transmitted to theisolated structure.

[0004] For effective protection of the structure it is thereforenecessary to use isolation methods to shift the natural frequency of thesystem consisting of the structure and the isolation means to a valuebelow 1 Hz.

[0005] The most widely used system for seismic isolation is to placeseveral elastomeric bearings on a foundation and build the structure onthese bearings. The seismic isolation bearings most commonly used todayare a multilayered laminated composite with alternating layers of rubberand steel plates. The steel plates provide a lateral constraint of therubber when the bearing is subjected to vertical load, but they do notinfluence the horizontal shearing behaviour of the rubber bearing.Therefore it is possible to produce bearings, which are very stiff tovertical applied loads, but very flexible in horizontal direction sothat the natural frequency of the building can be shifted appropriately.

[0006] An alternative design of such bearings is disclosed in U.S. Pat.No. 5,904,010 (Javid et al.). Javid shows an elastomeric bearingcomprising a stacked series of elastomeric laminae forming a unit cellhaving a stacked height corresponding to an overall horizontal andvertical stiffness of the unit, the laminae being in a vulcanizedadherent connection with each other; and wherein at least one of thelaminae includes a series of pretensioned continuous fibers extendingacross opposite side edges of at least one laminae. Structures supportedby such elastomeric elements are efficiently protected from the dangerof earthquakes. However, it is a rather expensive way to use suchelements since the costs of producing these elements are high and thestructures have to be adapted in order to be connected to theseelements.

[0007] Especially for structures like oil tanks or other groundsupported liquid filled structures, which have in general a very thinbottom surface and are placed directly on the prepared natural gradewithout proper foundation, the use of seismic bearings is problematic.In fact, the use of discrete, relatively small sized seismic bearingswould require the construction of a stiff support structure in order totransmit the uniform distributed load (pressure) from the bottom plateto a finite number of seismic bearings. Furthermore, mechanicalconnectors (e.g. bolts) are needed to permanently connect individualbearings to the tank or to a supporting structure. Similar connectionsare needed to fix the bottom plate of the bearings with a supportingconcrete like rigid foundation.

SUMMARY OF THE INVENTION

[0008] The objective of the present invention is to provide an improvedmethod to manufacture and build seismic protection systems forstructures subjected to seismic ground motion. The present inventionalso allows improvements in the design, manufacture and efficiency ofenergy dissipating devices in order to minimize the transfer of forcedvibration from manufacturing equipment, trains and subways to thesurroundings. Improvements are given by:

[0009] Reduce the weight of the isolation system;

[0010] Reduce manufacturing costs;

[0011] Reduce transportation costs;

[0012] Reduce installation costs;

[0013] Increase flexibility in the design concepts.

[0014] The method of the invention comprises the following steps:

[0015] preparing a horizontal surface, which may be the properlymodified natural grade in the case of liquid filled structures or acontinuous foundation in case of building like structure;

[0016] providing flexible damping systems each consisting essentially ofa layered composite of unreinforced isotropic rubber sheets andfiber-reinforced elastomeric slabs. These layers are stacked on top ofeach other in order to satisfy proper vertical and horizontal stiffnessrequirements;

[0017] the layered damping elements will cover the same surface areas asthe supported liquid storage tank or the supported continuousfoundation;

[0018] The supported structure is not physically connected to thedamping elements, thus no anchorage system is required;

[0019] The individual layers comprising the damping element are notvulcanised or permanently connected. The generated friction forcebetween them is sufficient to eliminate any slipping.

[0020] It is a significant aspect of this invention that there is noneed to provide a large number of single isolation bearings with adistance one to the other, but to cover the substantially entire bearingsurface with several damping layers arranged next to each other, ifneeded by cutting prefabricated slabs. In the same way additional layersare stacked on top of each other to reach the vertical and horizontaldesign stiffness. The structure is supported by the upper surface of theuppermost layer of these damping elements. Therefore it is possible tobuild for example an oil tank on the damping elements without providingfor a thick concrete base or any other anchorage system. Since thedamping layers are flat it is not necessary to connect the single rubbersheets and fiber reinforced composites by vulcanizing or glueing. It isa simple way to roll the damping elements off from a roll or coil andcutting them into the required dimension in situ. The fibers constrainthe rubber in horizontal direction so that vertical stiffness isattained.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 shows schematically a layered isolation system of theinvention installed directly on the prepared natural grade forprotecting an oil tank in an axonometric view;

[0022]FIG. 2 is a detail of an isolation system showing the method ofproduction;

[0023]FIG. 3 is a view similar to FIG. 1 showing a building supported byan isolation system of the invention on top of a continuous solidfoundations;

[0024]FIG. 4 shows an isolation system for bearing a rail track;

[0025]FIG. 5 shows a machine based upon an isolation system of theinvention;

[0026]FIG. 6 is an explanation of the method of the invention in anaxonometric view.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027]FIG. 1 shows the use and assembly of a typical layered isolationsystem 2 for the protection of liquid filled structures such as acylindrical tank 1. During an earthquake the ground 3 will be subjectedto strong ground motion and the layered isolation system will decouplethe superstructure 1 in order to minimize damage.

[0028] This isolation system may be located directly between theprepared natural grade 3 and the bottom plate 4 of the tank 1. Theisolation system 2 consists of several layers of unreinforced rubberslabs 5 and fiber-reinforced elastomeric mats 6 stacked on top of eachother.

[0029] A large contact area exists between the tank bottom 4 and theuppermost rubber laminae 5, 6, as well as between individual layers 5,6. In this particular case sufficient friction develops to ensure thatno slipping takes place. There is no need to use any form of mechanicalconnector between the natural grade 3, the isolation system 2 and thebottom surface 4 of the superstructure 1. In order to increase thefrictional contact, the uppermost elastomeric laminae 5, 6 may beconnected to the superstructure 1 using a strong epoxy bond.

[0030]FIG. 2 shows that fiber direction may change between one layer tothe next in case an anisotropic design of the isolation system is askedfor. The layers 7 a, 7 b to assemble the final isolation system 2consist of fiber-reinforced elastomeric mats 8 wherein each mat 8consisting of a reinforcing layer 10 sandwiched between rubber layers 9.

[0031]FIG. 3 shows the use of the proposed layered isolation systemlocated between a rigid continuous or non-continuous foundation 11 and asuperstructure e.g. a building 12. A large contact area is necessary toensure sufficient frictional forces to eliminate or minimize slipping.The elastomeric layers are reinforced by using fibers in both in-planedirections.

[0032]FIG. 4 shows the layered isolation system to reduce energytransferred to the ground from traffic-induced vibrations. FIG. 4 showssleepers 13 of a train truck 14 mounted on a rigid concrete slab 15. Theisolation system 2 is located between the rigid slab 15 and the naturalgrade 3, or between rigid slab 15 and a concrete foundation. Fibers inthe elastomer slabs may have a preferred direction or be installed inorder to have transverse isotropy.

[0033] Another application of the isolation system is depicted in FIG.5. It shows the isolation of vibrating equipment, e.g. a machine 16 orany other induced forced vibration by installing the isolation system 2between the bottom surface 17 of the equipment and the rigid foundation,e.g. the floor of a manufacturing hall.

[0034] Further FIG. 6 shows a schematic representation of the in-situassembly of the isolation system. Individual reinforced layers eachconsisting of an unreinforced rubber slab 5 or a fiber-reinforcedelastomeric mat 6 will be unrolled at the construction site and used tocover the area underneath the superstructure. The first layer offiber-reinforced material may lie directly on the prepared natural gradeor on a concrete-like foundation. The fiber-reinforced slabs may changedirection in each layer in order to increase the friction contact. Novulcanization is needed to permanently connect individualfiber-reinforced slabs, in special cases a strong epoxy bond may beused. In general, due to the large contact area and due to the permanentvertical load, sufficient friction will exist to ensure that no slidingtakes place.

What is claimed is:
 1. A method for protecting a structure from seismicground motion and forced vibration comprising the following steps:preparing a horizontal bearing surface or a solid foundation on top ofwhich the isolation system will be placed; providing flexible dampingelements of a group consisting of unreinforced rubber slabs andfiber-reinforced elastomeric mats; each mat consisting essentially of apiece of a rubber mat reinforced with fibers extending parallel to thesurfaces of the mat; covering the horizontal bearing surface or thesolid foundation with several layers of damping elements, each layerconsisting of a plurality of elongated pieces of the mat arranged sideby side; erecting the structure by putting the lower surface of thestructure directly upon the upper surface of an uppermost layer ofdamping elements.
 2. A method of claim 1, wherein the layers ofunreinforced rubber slabs and fiber-reinforced elastomeric mats are notconnected to each other.
 3. A method of claim 1, wherein dampingelements of one layer are not mechanically or chemically connected todamping elements of an adjacent layer.
 4. A method of claim 1, whereinthe damping elements are obtained by rolling off from a roll or coil andcutting them to the required dimension in situ.
 5. A method of claim 1,wherein between two layers of unreinforced rubber slabs at least onefiber-reinforced elastomeric mat is provided.
 6. A method of claim 1,wherein several fiber-reinforced elastomeric mats are stacked directlyon top of each other.
 7. An elastomeric seismic isolation system forbearing a structure supported by a foundation comprising: a first layerof flexible damping elements each consisting essentially of a piece of arubber slab reinforced with fibers extending parallel to the surfaces ofthe mat disposed on a horizontal bearing surface on the top of thefoundation or a prepared natural grade; at least one intermediate layerof flexible damping elements each consisting essentially of a piece of arubber mat reinforced with fibers extending parallel to the surfaces ofthe mat disposed on an upper surface of a previous layer of flexibledamping elements; an uppermost layer of flexible damping elements eachconsisting essentially of a piece of a rubber mat reinforced with fibersextending parallel to the surfaces of the mat disposed on an uppersurface of a previous layer of flexible damping elements and disposedfor bearing a lower surface of the structure.
 8. An elastomeric seismicisolation system of claim 7, wherein damping elements of one layer arenot interconnected one to the other.
 9. An elastomeric seismic isolationsystem of claim 7, wherein damping elements of one layer are notconnected to damping elements of an adjacent layer.
 10. An elastomericseismic isolation system of claim 7, wherein the structure is an oiltank supported directly by the prepared natural grade without concretefoundations.
 11. An elastomeric seismic isolation system of claim 7,wherein each layer covers an area, which is essentially equal to an areaof the lower surface of the structure.
 12. An elastomeric seismicisolation system of claim 7, wherein the damping elements have anessentially longitudinal form with a longitudinal axis and that thelongitudinal axis of the damping elements of one layer are parallel oneto the other.
 13. An elastomeric seismic isolation system of claim 12,wherein the longitudinal axis of the damping elements of at least twoadjacent layers are oriented in angle.
 14. An elastomeric seismicisolation system of claim 13, wherein said angle is within a rangebetween 45° and 90°.
 15. An elastomeric seismic isolation system ofclaim 7, wherein between two layers of unreinforced rubber slabs atleast one fiber-reinforced elastomeric mat is provided.
 16. Anelastomeric seismic isolation system of claim 7, wherein severalfiber-reinforced elastomeric mats are stacked directly on top of eachother.